Packaging material, e.g., packaging material having a core layer of cellulose such as paper, paperboard, cardboard, etc., is normally laminated into a multi-layer structure for providing superior properties of the final package. In liquid food packaging it is e.g. desired to provide a robust package preventing leakage, and in some cases it may also be desired to prevent the enclosed product to be exposed to oxygen. These and other requirements of the final package have pushed the development of packaging material in the form of multi-layer laminates, wherein a core layer of e.g. carton is covered by plastic layers on both sides.
One well known layer structure of a packaging laminate 10 is shown in FIG. 1, according to which a core layer 11 of carton (hereafter also referred to as ‘carton layer 11’) is covered by a plastic layer 12 of low-density polyethylene (LDPE) for forming an outside of the packaging laminate 10, i.e. a side of a final package that is not in contact with an enclosed product, usually a liquid food product, such as milk, dairy, beverage, juices, puree, etc. An inner side of the carton layer 11 is covered with a similar layer 13 of LDPE (hereafter referred to as ‘LDPE bonding layer 13’) which is used to laminate, i.e. bind together, a thin foil of aluminium 14 (hereafter also referred to as ‘aluminium foil 14’) to the carton layer 11. In order to avoid contact between the enclosed product and the aluminium foil 14 the aluminium foil 14 is covered by a polymer layer, normally first an adhesive polymer layer 15 (hereafter also referred to as ‘layer 15’ or ‘inner layer 15’). The layer 15 is in turn covered by an innermost layer 16 of LDPE or a blend of LDPE with an linear low density polyethylene (LLDPE), such as an LLDPE polymerised in the presence of a constrained geometry catalyst, such as a metallocene-catalyst (m-LLDPE) (hereafter also referred to as ‘layer 16’ or ‘inner layer 16’) which is to be in direct contact with the enclosed product in a packaging container.
The above-mentioned example of a packaging laminate has proven to be generally very well functioning for enclosing liquid food products in an aseptic manner.
However, the aluminium foil 14, which forms an oxygen barrier is normally very thin, such as in the range from about 5 to about 12 μm, such as from 5 to 10 μm. For this reason, the aluminium foil 14 is rather delicate and may cause some concerns when the LDPE bonding layer 13 and the inner layers 15 and 16 are being laminated to the aluminium foil. Some of the concerns arise from bad adhesion between the adjacent layers, uneven application of the LDPE bonding layer 13, or of the inner layers 15 and 16, and defects in the laminate layer, such as for example very small holes, so-called pin-holes. For example, there may be zones within the layered laminate that have less reliable adhesion, such as around crease lines or folding lines, that have been impressed into the carton layer 11, before lamination. Furthermore, there may be breakage of the aluminium foil 14 in the areas near pre-cut holes of the core layer, i.e. the carton layer, due to tensions or wrinkles in the aluminium foil, formed during lamination. The breakages in the aluminium foil may occur first during subsequent handling in the filling and packaging machines or during distribution and transport, when the pre-stressed aluminium foil material is subject to further stress. Such concerns may thus be more pronounced when the carton layer 11 has deviations from a flat, evenly thick, layer, such as pre-cut holes, slits or attenuations, i.e. features that are provided in the carton layer 11 before the lamination operations, and further at high web speeds in the lamination and coating operations.
The application of the polymer layers in the laminate is conventionally done by extrusion coating or extrusion lamination of molten thermoplastic polymer. In the extrusion lamination operation, the laminate bonding layer 13 is extruded from a die in the form of a molten curtain which is applied between the paperboard web and the web of aluminium foil 14, while they are entering the nip between a cooling roller and a press roller. Similarly, in an extrusion coating operation, the innermost layer 16, together with the adjacent inner layer 15, is extruded from a die in the form of a molten curtain which is applied onto the laminated web of carton 11 and aluminium foil 14, while they are entering another nip between a cooling roller and a press roller. The temperature of the molten polymer curtain is rather high when applied to the aluminium foil 14, such as in the range of 200-340° C., and when the molten film solidifies to form the laminate layer 13, its density changes by at least 10%. Since the aluminium foil is to the major part supported by the more dimensionally stable web of carton, this does not normally cause any concern.
In the areas of any pre-cut holes of the paperweb, however, the aluminium foil is unsupported. Since the molten web is hot at application to the unsupported aluminium foil 14 in these areas, there may be tensions or wrinkles in the aluminium foil 14 due to polymer shrinkage as the polymer cools down and solidifies around the edges of the pre-cut hole. This may lead to unwanted effects, such as inclusion of air and tensions and wrinkles in the thin and sensitive aluminium foil layer in the final laminate, during the high speed process of melt extrusion laminating, or extrusion coating, of the aluminium foil with the adjacent layers.
Thus, in view of the above there is still a need for an improved and more robust and stable method for manufacturing of a laminated packaging material, which method allows for providing laminated packaging materials with good and reliable adhesion between thin and sensitive laminated material layers, also at high lamination and web forwarding speeds, and also in difficult areas of the laminated material. There is particularly a need, around the paperboard hole regions of a paperboard, to avoid and minimise the formation of wrinkles, tensions or air entrapments. In particular, the membrane, i.e. the laminated aluminium foil and polymer layers within the pre-cut hole region of the paperboard, should have good adhesion between the layers throughout the area of the membrane or hole, and the membrane should have as few defects as possible, i.e. tensions or wrinkles in the aluminium foil, air entrapments, breakages or pin-holes of layers, uneven thickness of the polymer layers, or other related defects.